Adaptation of the Coulter Counter to the study of adipose cellularity has provided an important tool for evaluating the relative contribution of size and number of adipose cells to lipid deposition in the body. With this technique, adipose cellularity has been investigated during normal growth and development and during onset of obesity in many different animal models and in different depots within each model (for recent reviews see 1-5). Although conditions have been identified in which adipose cells increase in number, it remains to be determined whether this increase represents lipid filling of differentiated cells or of cells recently differentiated from a pool of mitotically ac-tive cells. The ability to identify the adipose precursor cell, or preadipocyte, and the factors that influence its proliferation and differentiation would greatly aid the quest to understand, prevent, and cure obesity in humans. Even though the proliferating preadipocyte has yet to be identified as a separate, distinguishable cell type, much has been learned from examination of gross-structural and non-adipocyte aspects of adipose tissues and from recent advances of our understanding of hormonal, nutritional, and genetic effects on the development of adipose tissue. The body of information relevant to the search for the adipocyte precursor cell and factors that promote its differentiation are the subject of this review. Studies that examined the gross-structural and nonadipocyte aspects of adipose tissue have largely been overlooked in recent years. In particular, the research of Wasserman (6-9), Liebelt (10-13) and Tedeschi (14-24) has pointed out the stromal-vascular and non-adipocyte aspects of adipose tissue in the fetus and in the growing and mature animal. This review will emphasize those studies that considered these aspects of white adipose tissue (WAT) development. Attempts to completely characterize adipocyte development (from presumptive to proliferating to mature differentiated cell) must be understood in terms of the development of all elements of adipose tissue at all levels of organization (from cell to tissue to depot). By studying lipid deposition at all levels of organization, new insight into the nature of the preadipocyte can be appreciated.